The invention relates generally to imaging systems. In particular, the invention relates to thin film transistors for use in detectors of such imaging systems and methods of making the same.
Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of a person or object that are otherwise inaccessible for visual inspection. For example, non-invasive imaging techniques are commonly used in the industrial field for inspecting the internal structures of parts and in the security field for inspecting the contents of packages, clothing, and so forth. One of the best known uses of non-invasive imaging, however, is in the medical arts where these techniques are used to generate images of organs and/or bones inside a patient which would otherwise not be visible.
One class of non-invasive imaging techniques that may be used in these various fields is based on the differential transmission of X-rays through a patient or object. In the medical context, a simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays. Typically the X-ray detection process utilizes a scintillator, which generates optical photons when impacted by X-rays, and an array of photosensor elements, which generate electrical signals based on the number of optical photons detected.
Some X-ray techniques utilize very low X-ray flux so that patient exposure can be extended. For example, fluoroscopic techniques are commonly used to monitor an ongoing procedure or condition, such as the insertion of a catheter or probe into the circulatory system of a patient. Such fluoroscopic techniques typically obtain large numbers of images that can be consecutively displayed to show motion in the imaged area in real-time or near real-time.
However fluoroscopic techniques, as well as other low X-ray flux imaging techniques, may suffer from poor image quality due to the relatively weak X-ray signal relative to the electronic noise attributable to the detector. As a result it is typically desirable to improve the efficiency of the detection process, such as by reducing the electronic noise attributable to the detector. For instance, various aspects of the thin film transistors (TFTs) employed to read out the detector elements may contribute to the overall electronic noise. For example, the source and drain of the TFT may be formed from the same layer of material as the data lines to which they connect. However, different electrical properties are typically desired in the data lines as compared to the source and drain and, therefore, configurations which improve data line performance may detriment source and drain performance and vice versa. For example, it may be desirable for the data lines to be relatively thick to reduce their resistance. However, thick source and drain lines will undercut during the wet etch performed during TFT formation, leading to a longer channel length. In general, the longer the channel, the greater the problems associated with electronic noise, which is a function of charge trapping which in turn is a function of the channel length. Additionally, the “on” resistance of the TFT is also a function of channel length resulting in slower pixel readout rates as channel length is increased. As a result, smaller channel lengths are generally desirable for improved TFT performance. Because of these contrary aspects, the data lines and/or the source and drain electrodes formed from the same layer of material (and, therefore, having the same thickness) may not have optimal performance.
Similarly, the scan line connected to the gate of the TFT may impact detector performance. In particular, the scan lines and gate are typically formed from same layer of material. However, as with the data lines, a thick scan line may be desirable to decrease the resistance of the scan line. The resulting thick gate electrode, however, may lead to leakage currents due to a larger step at the edge of the gate electrode. Therefore, improving the electrical properties of the readout lines (i.e., the scan lines and data lines) may inadvertently degrade detector performance in other ways due to the negative impacts on electrodes formed from the same respective layers of materials.
Therefore, there is a need for a thin film transistor that addresses some or all of the problems set forth above.